1. Field of the Invention
The present invention relates to an electromagnetic wave shielding sheet which, when used, is placed on the viewing side of an electromagnetic device such as a display and, in this state, can shield electromagnetic waves and enables the electromagnetic device such as a display to be seen through the sheet.
2. Background Art
It has been pointed out that electromagnetic waves generated from electromagnetic devices adversely affect other electromagnetic devices or human body and animals. To avoid the adverse effect, various electromagnetic wave shielding means have been demanded. In particular, since plasma displays (hereinafter referred to as “PDPs”) generate electromagnetic waves with frequencies of 30 to 130 MHz which often adversely affect computers and peripheral devices of computers, minimizing the leakage of electromagnetic waves generated from PDPs to the outside of the PDPs is required.
Conventional means for electromagnetic wave shielding include, for example, a method wherein the electromagnetic device is covered with a case made of a high electrically conductive material and a method wherein the electromagnetic device is covered with an electrically conductive net. These methods, however, sacrifice see-through properties of the electromagnetic device and thus are not suitable for devices where viewing is necessary. On the other hand, electromagnetic wave shielding means composed of a transparent indium tin oxide (hereinafter referred to as “ITO”) film provided on a transparent film has been developed. The ITO film has a high level of see-through properties, but on the other hand, the electrical conductivity is so low that the electromagnetic wave shielding capability is poor. As a result, the use of this means has been limited only to devices which generate no significant amount of electromagnetic waves.
On the other hand, a sheet having a combination of electromagnetic wave shielding capability with see-through properties has been developed. This sheet is produced by etching a metal foil stacked onto a film to densely form openings and, thus, to render the metal foil mesh. Further, in this type of sheet, an improved sheet has been provided in which the thickness of the metal foil and the dimension of the mesh have been made proper, the capability of shielding the same level of electromagnetic waves as the level of the electromagnetic waves generated from PDPs has been imparted, and the visibility of the display screen has been improved.
The mesh metal foil formed by etching is difficult to be solely handled. For this reason, for the formation of the mesh metal foil, it is common practice to adopt a method wherein a laminate composed of an untreated metal foil stacked onto a film through an adhesive layer is formed and the metal foil in the laminate is etched to form a pattern.
When etching is carried out with an iron chloride- or copper chloride-containing solution as a conventional etching liquid, however, the adhesive layer is often colored light yellow. Thus, in some cases, the formation of a colorless, transparent etched sheet is difficult. When this sheet is used in such a state that the sheet is placed on the viewing side of PDP, the colored adhesive layer functions like a color absorption filter and often deteriorates blue luminance of PDP.